Convenient gel-processing techniques have been developed for polyethylene to provide articles (e.g. fibers and films) of good mechanical properties. See, e.g., U.S. Pat. No. 4,422,993, which is hereby incorporated in its entirety by reference. However, efforts to employ gel-processing techniques to provide polypropylene articles of good mechanical properties have, at best, only seen moderate success and/or lead to cumbersome processing schemes. See, e.g., Bastiaansen et al., “Makromol Chem Macromol. Symp.” 1989, 28:73; Peguy et al., “Polym Commun”, 1984, 25:39; Matsuo et al., “Polym J”, 1986, 18:759; Ohta et al., “Polymer”, 1998, 20:4793; and U.S. Pat. No. 4,413,110. Particularly, the processing techniques appear to be impractically slow, therewith preventing fiber production on an industrial scale.
For instance, referring to the literature mentioned above, Peguy et al. describes the formation of gel-films by quenching hot solutions of polypropylene (hereinafter also referred to as “PP”) in decalin to −25° C. and leaving the material for several hours. Ohta et al. mentions the method of pressing “gel-like spherulites”, which were prepared at a cooling rate as low as 1.5° C./min. U.S. Pat. No. 4,413,110 reports producing gel-fibers of PP by extruding 8 wt % solutions of the polymer in paraffin oil and extracting the latter with trichlorotrifloroethane at a speed of only about 6 m/min.
The lack of a convenient process to convert PP into high-performance products is unfortunate because, even though the theoretical ultimate stiffness and strength of PP are mentioned to be somewhat lower than those of polyethylene (e.g., Kausch in “Polymer Fracture”, Springer Berlin, 1992, p. 6), the significantly higher melting temperature of PP, especially when constrained (Bastiaansen et al. in “Makromol. Chem. Macromol. Symp.”, 1989, 28:73), would provide distinct advantages over polyethylene.
References mentioning employing polyolefins in the formation of porous membranes include U.S. Pat. No. 6,632,850 (which is hereby incorporated in its entirety by reference); JP-A-08012799; JP-A-05222236; and JP-A-09003226.
The article by Kristiansen et al. in the journal “Polymer”, vol. 44, p. 5885-5891 (2003) mentions isotactic polypropylene and the nucleating agent DMDBS.
Objectives of the present invention include providing polymer gel-processing techniques with reduced gelling times and providing polymer articles of high modulus.